1. Field of the Invention
The present invention relates to a heat sensor, and, more particularly to a compensation type heat sensor having a constant temperature function capable of detecting a fire by detecting the actual rise in temperature and a differential function capable of detecting, for example, an initial fire by detecting a rapid rise in the temperature.
2. Related Art Statement
Hitherto, a structure formed as shown in FIG. 11 has been known as a compensation type heat sensor of the aforementioned type. The sensor has both a constant temperature function realized by using a semiconductor thermistor device, the resistance value of which is changed depending upon the change in the temperature, so as to transmit alarm information when the temperature of a supervisory region has been raised to a predetermined dangerous level and a differential function for transmitting alarm information after it has detected a fact that the temperature of the supervisory region has been rapidly raised at a rate higher than a predetermined rate of rise in the temperature.
Referring to FIG. 11, the structure and the operation of the compensation type heat sensor will now be described. A positive connection terminal P1 and a negative connection terminal P2 are connected to a transmitting line extending from a receiver (omitted from illustration) disposed in a central supervisory room or the like. The aforesaid sensor receives electric power supplied from the receiver via the transmitting line and as well as transmits fire alarm information to the receiver. A constant voltage circuit 13 such as the three-terminal regulator connected to a position between the aforesaid connection terminals P1 and P2 forms a power supply voltage VDD capable of supplying predetermined voltage. The power supply voltage VDD acts to operate the following fire detection circuit.
Reference numeral 1 represents a thermistor device having a negative temperature coefficient with which the resistance value thereof is decreased as the temperature is raised. Reference numeral 2 represents a resistor having a predetermined resistance value. At a junction of the aforesaid elements 1 and 2, voltage VC1 is generated so as to be supplied to a non-reversal input contact of a comparator 5.
Reference numerals 3 and 4 represent resistors each having a predetermined resistance value. At a junction of the two resistors 3 and 4, reference voltage VR1 is generated so as to be supplied to a reversal input contact of the comparator 5.
If the aforesaid voltages hold a relationship VC1&lt;VR1, the level of output signal of the comparator 5 becomes a logical value level "L". If a relationship VC1&gt;VR1 is held due to the fact that the ambient temperature has exceeded the predetermined dangerous level, the level of the output signal Q5 becomes a logical value level "H".
Reference numeral 6 represents a reverse current prevention diode connected to an output contact of the comparator 5. The output signal Q5 transmitted from the comparator 5 and the diode 6 is supplied to a gate contact of a thyristor device 16 via voltage-dividing resistors 14 and 15. The signal denoting the level "H" of the output signal Q5 becomes a trigger gate signal for turning on the thyristor device 16.
The thyristor device 16 has the anode contact connected to the positive connection terminal P1 and the cathode contact connected to the negative connection terminal P2. When the thyristor device 16 is turned on in response to the aforesaid trigger gate signal applied to the position between the gate and cathode contact, it lowers the impedance between the connection terminals P1 and P2 to transmit fire generation information to the receiver.
Then, the operation of a control circuit constituted by the aforesaid devices will now be described with reference to FIG. 12. If the ambient temperature has been raised as shown in FIG. 12, the voltage VC1 is raised as the temperature is raised as shown in FIG. 12B. If the voltage VC1 exceeds the reference voltage VR1 at certain time T0, the output voltage Q5 of the comparator Q5 becomes the level "H" as shown in FIG. 12C. Simultaneously, voltage 16 between the connection terminals P1 and P2 is lowered, so that the fire alarm information is transmitted.
The resistance values of the resistors 2, 3 and 4 and the temperature characteristic of the thermistor device 1 are respectively determined so as to make the voltage VC1 and the reference voltage VR1 to be the same when the ambient temperature has been raised to temperature TR1 at which a discrimination must be made that a fire has been taken place.
Since the sensor circuit constituted by the elements 1 to 6, as described above, transmits alarm information when the ambient temperature has exceeded the predetermined dangerous level TR1, the circuit shown in FIG. 11 has the constant temperature function.
Further referring to FIG. 11, reference numerals 7 and 8 represent thermistor devices each having negative temperature characteristics. Voltage VC2 generated at their junction is supplied to a non-reversal input contact of the comparator 11, the two thermistor devices 7 and 8 having different heat responses. That is, the thermistor device 7 has a small heat time constant and thereby has quick heat response. On the other hand, the thermistor device 8 has low heat response because it has a large heat time constant. Therefore, if the temperature is rapidly raised, resistance value R7 of the thermistor device 7 is decreased quicker than resistance value R8 of the thermistor device 8. The voltage VC2 is rapidly raised when the resistance ratio R7/R8 is rapidly changed. The voltage VC2 divided by the pair of the thermistor devices 7 and 8 having different characteristics is, as described above, considerably changed when the ambient temperature has been rapidly changed. Therefore, the circuit 11 has a function to serve as a sensor capable of detecting a rapid rise in the temperature. In a case where the temperature is raised slowly, the resistance of the thermistor devices 7 and 8 become substantially the same. As a result, the resistance ratio R7/R8 becomes substantially constant and thereby the change in the voltage VC2 due to the rise in the temperature is extremely reduced.
Reference numerals 9 and 10 represent resistors each having a constant resistance. Reference voltage VR2 generated at their junction is supplied to a reversal input contact of the comparator 11.
If a relationship VC2&lt;VR2 is held, output signal Q11 of the comparator 11 becomes the logical value level "L". On the other hand, the output signal Q11 becomes the logical value level "H" when a relationship VC2&gt;VR2 is held because the ambient temperature has been rapidly raised due to a fire.
Reference numeral 12 represents a reverse-current prevention diode connected to an output contact of the comparator 11. The output signal Q11 transmitted from the comparator 11 and the diode 12 is supplied to the gate contact of a thyristor device 16 via the voltage-dividing resistors 14 and 15.
A sensor circuit constituted by the elements 7 to 12 has the aforementioned differential function which acts at this time. Hence, by turning on the thyristor device 16 at the time at which the output signal Q11 has become the logical value "H" to lower the impedance between the connection contact P1 and P2, alarm information is transmitted.
As described above, the conventional compensation type heat sensor has the constant temperature function for detecting a fire by detecting the actual rise in the temperature and the differential function capable of detecting, for example, the initial fire by detecting a rapid rise in the temperature, so that the fire detection can be performed basing on the actual fire generation mechanism.
However, the conventional compensation type heat sensor is constituted by individual circuits for corresponding functions. Therefore, there arises problems in that the overall cost and the size of the overall body of the sensor cannot be reduced because a too large number of parts must be used.